In Reply: Drs Gau, Clay, and Carlsen note many factors that are known to affect serum 25-hydroxyvitamin D concentration.1 Indeed, we have reported these same effects on serum 25-hydroxyvitamin D in our study cohort.2 Serum PTH levels are also affected by similar factors and, in our present study, we did enter these as covariates for serum PTH, our dependent variable. Therefore, we do not see it appropriate to also adjust serum 25-hydroxyvitamin D with respect to these variables, as our main study question relates to the significance of absolute levels of serum 25-hydroxyvitamin D.